Police investigating the disappearance of 19-year-old Belinda Peisley in 1998 mishandled the case and missed a critical window of opportunity to gather evidence into her suspected homicide, a former NSW Police officer and missing persons expert says.

Karen Karakaya worked as a police officer in the NSW Coronial Support Unit and Missing Persons Unit in the late 1990s, during the time the young mother vanished from Katoomba in the Blue Mountains, west of Sydney.

Ms Karakaya told the ABC's true crime podcast investigating the disappearance of Ms Peisley, Unravel, it was "surprising" NSW police marked Ms Peisley's missing person's report as needing "no further investigation" just four days after it was officially made.

"You would not expect to see 'no further investigation' on something that required investigation," Ms Karakaya told Unravel.

"Those first few days and weeks are really critical to the investigation … It's a shame that this has happened."

"The only time you would see that this early on is if [Ms Peisley] was located, and they were locating and writing off the report as no further actions required.

"It's just so sad that these things happen — that someone's out there that's missing."

At the coronial inquest into the disappearance of Ms Peisley, held over three weeks in 2012 and 2013, the officer who took Ms Peisley's missing person's report, Matthew U'Brien, agreed that in retrospect there were obvious steps he could have taken, but didn't, like visiting Ms Peisley's residence and interviewing her friends and associates.

However, the lack of police action at the time wasn't solely Mr U'Brien's responsiblity; the priority given to particular cases wasn't something decided by individual officers — it was also up to their superiors.

"The police never treated this as a potential homicide," Mr Strickland said.

"They treated this as a junkie who had gone missing, and so the police didn't do any proper forensic sampling, particularly of blood samples that were in Belinda's house, and they didn't interview a number of critical witnesses.

"It is known that it is the first part of the investigation [that] is the critical one, and if you don't do that right, then it's often very hard to pick up the pieces later."

At the conclusion of the inquest into Ms Peisley's suspected death, coroner Paul McMahon recommended that standard operating procedures in missing persons cases be changed if a missing person was deemed to be of high or very high risk of having been the victim of a homicide or suspicious death the Homicide Squad is immediately advised.

Mr McMahon also recommended that the Homicide Squad should, if it considers it appropriate, lead the investigation in the first 72 hours.

It was also recommended that the investigation into Ms Peisley's suspected death be referred to the NSW Police Unsolved Homicide Unit.

Ms Peisley's suspected homicide was one of 500 cold cases to be re-examined by the NSW Police's Unsolved Homicide Squad last year, and in December 2018 a forensic excavation of Ms Peisley's home uncovered three pieces of clothing and underwear that are being forensically tested.

Key dates in the Belinda Peisley case

September 1997

Ms Peisley inherited about $150,000 from a relative.

March 1998

Ms Peisley bought a house on Trow Avenue in Katoomba for about $118,000. She moved in with her three-year-old son, Cody. Her other son Billy lived with his father in Sydney.

Over the course of the next few months, the house became like a "drop-in centre" for the local drug community, according to evidence given at the coronial inquest into Ms Peisley's disappearance. Her heroin use also increased over this time.

July 1998

Ms Peisley began a new relationship with Jason (whose name has been changed for legal reasons). By this time, she had spent almost the entirety of her inheritance.

September 26 1998

This is the last day Ms Peisley is known to have been alive.

Ms Peisley attended a gathering in Katoomba where she was punched in the face by an acquaintance.

After the altercation, Ms Peisley allegedly got a taxi home with her boyfriend, Jason (whose name has been changed for legal reasons). He said they had an argument at her house and she smashed mirrors and windows.

Jason left to allegedly stay at a friend's house. This friend remembered Jason coming over that night, but not staying over.

Ms Peisley's neighbour called the police after hearing yelling and things smashing. Police arrived at Ms Peisley's house and found her intoxicated and alone. They took her to a hospital in Katoomba where she was triaged by a nurse. She had a cut on her right hand.

Ms Peisley left the hospital at about 8:50pm before a doctor saw her. This was the last reported sighting of her alive.

At about 10:30pm, Ms Peisley called her mother, Lesley, and asked her to bring her son, Cody, to her home at Trow Avenue (Cody was staying with Lesley). Lesley said it was too late and they would come in the morning.

September 27 1998

Heidi Wailes said after she heard about her friend Ms Peisley being punched, she went to her house to see how she was, but no-one was home. Ms Wailes said she went inside the house to look for Ms Peisley and found her bag, with her wallet inside, squashed down the back of the couch. Ms Wailes did not make an official report to police about this until a considerable time later.

Jason (whose name has been changed for legal reasons) went to the NRL Grand Final in Sydney with several people.

September 28 1998

Jason returned to Ms Peisley's house in the morning and found the front door open and windows smashed. He looked around for Ms Peisley but she wasn't there. He said he found her keycard and Medicare card, and he took them. He waited for her for a couple of hours, then left.

A Department of Community Services (DoCS) worker went to Ms Peisley's home to visit her. She saw windows were smashed, and there was no answer at the door.

A note on the police reporting system said Jason had informed police Ms Peisley had gone "berserk" at her house on September 26, smashing property and windows and throwing him out. The note also said Ms Peisley had not been seen at the house for two days and her current whereabouts were unknown.

September 29 1998

The DoCS worker returned to Ms Peisley home to ascertain her whereabouts but nobody was there. The worker notified police about her concerns for Ms Peisley's welfare.

Police attend Ms Peisley's home and made a forced entry into the property. One officer who attended later remembered seeing what appeared to be blood in the bathroom.

October 6 1998

Ms Peisley's mother officially reported her as missing to police. A photograph of Ms Peisley was collected by police and posted on an information board in Katoomba.

October 10 1998

4 November 1998

Police were informed of activity in Ms Peisley's bank account and the possibility that Jason (whose name has been changed for legal reasons) was the person accessing it.

16 November 1998

Police took their first formal statement from Jason regarding Ms Peisley's disappearance. Jason admitted to using her keycard to withdraw money from her account, but said she owed him money.

8 October 2012

The inquest into the disappearance and suspected death of Ms Peisley began at the NSW State Coroner's Court in Katoomba. The inquest continued over 15 days in 2012 and 2013 at Katoomba Local Court, Parramatta Local Court and the NSW State Coroner's Court in Glebe.

11 October 2013

NSW deputy state coroner Paul McMahon determined Ms Peisley died on or about 26 September 1998 in or around Katoomba, and her death was more likely than not the consequence of the action of a third party. The coroner was unable to make a finding as to the cause and manner of her death.

At the conclusion of the inquest, Mr McMahon found it was unlikely Jason (whose name has been changed for legal reasons) was involved in Ms Peisley's disappearance and suspected death. As to whether Ms Wailes had any direct knowledge of and/or involvement, the coroner said the evidence was inconclusive.

3 December 2018

Police conducted a forensic dig at Ms Peisley's Katoomba home and found three pieces of clothing that are being tested for DNA.